Polycarbonates have excellent heat resistance, impact resistance and transparency, and in recent years are widely used in many fields.
There have been many studies of polycarbonate production processes. In industry, polycarbonates derived from aromatic dihydroxy compounds, for example, 2,2-bis(4-hydroxyphenyl)propane (hereinafter “bisphenol A”), are produced by an interfacial polymerization method or a melt polymerization method.
In the interfacial polymerization method, a polycarbonate is produced from bisphenol A and phosgene, but poisonous phosgene must be used. Other problems are that the apparatuses are corroded by chlorine-containing compounds such as hydrogen chloride and sodium chloride byproducts, and methylene chloride used in a large amount as the solvent, and that it is difficult to remove impurities such as sodium chloride and residual methylene chloride and consequently polymer properties are adversely affected.
On the other hand, as a method for producing polycarbonates from an aromatic dihydroxy compound and a diaryl carbonate, for example, it has been known a melt polymerization method from long ago in which bisphenol A and diphenyl carbonate are polymerized in a melt state by transesterification, while removing aromatic monohydroxy compounds that are by-produced. The melt polymerization method has advantages such as the freedom from the use of solvents in contrast to the interfacial polymerization method. However, the polymer viscosity in the system is rapidly increased with the progress of the polymerization to make it difficult to remove aromatic monohydroxy compound byproducts efficiently from the system. As a result, this method has an essential problem that the reaction rate is markedly decreased and thus increasing the polymerization degree is difficult.
To solve this problem, various approaches have been studied to withdraw aromatic monohydroxy compounds from the highly viscous polymer. For example, Japanese Patent Publication No. S50-19600 discloses a screw polymerizer having a vent section, and Japanese Patent Application Kokai Publication No. H02-153923 discloses the combined use of a thin-film evaporator and a horizontal polymerizer.
U.S. Pat. No. 5,521,275 discloses a process in which an aromatic polycarbonate is redistributed in the presence of a catalyst under reduced pressure conditions using an extruder having a polymer seal section and a vent section.
However, the processes disclosed in the above publications cannot sufficiently increase the molecular weight of polycarbonates. The polymerization using large amounts of catalysts or involving high-shear severe conditions causes a poor hue of the resins or marked adverse effects on the resins such as the occurrence of crosslinking reaction.
In the melt polymerization method, it is known that the polymerization degree of polycarbonates is increased by adding polymerization accelerators to the reaction system. The ability to increase the molecular weight in a short reactor residence time and at a low reaction temperature enhances the output of polycarbonates and consequently facilitates designing a simple and inexpensive reactor.
EP Patent No. 0595608 discloses a redistribution process involving the reaction of diaryl carbonates. However, a significant increase in molecular weight cannot be obtained. U.S. Pat. No. 5,696,222 discloses a process for producing a polycarbonate with a high polymerization degree which includes the addition of a specific polymerization accelerator, for example, an aryl carbonate or dicarboxylate ester compound such as bis(2-methoxyphenyl) carbonate, bis(2-ethoxyphenyl) carbonate, bis(2-chlorophenyl) carbonate, bis(2-methoxyphenyl) terephthalate or bis(2-methoxyphenyl) adipate. This patent literature 5 teaches that the use of the ester compound as a polymerization accelerator introduces ester bonds and results in a polyester carbonate copolymer (instead of a homopolymer) exhibiting low hydrolysis stability.
Japanese Patent No. 4112979 discloses a process in which an aromatic polycarbonate having an increased molecular weight is obtained by using salicyl carbonates in the reaction.
Japanese Patent Kohyo Publication No. 2008-514754 discloses a polymerization process which includes introducing components including a polycarbonate oligomer and a bissalicyl carbonate to an extruder.
Japanese Patent No. 4286914 discloses a process in which an aromatic polycarbonate is reacted with an active hydrogen compound (a dihydroxy compound) to increase the amount of terminal hydroxy groups and thereafter a salicylate ester derivative is added to couple the aromatic polycarbonate molecules having an increased amount of terminal hydroxy groups.
Due to the need of increasing the amount of terminal hydroxyl groups of the polycarbonate, the process disclosed in the above patent literature 8 involves a step of the reaction with an active hydrogen compound and a step of the reaction with a salicylate ester derivative. In addition to this complexity of the steps, there is a risk that properties may be lowered because of the fact that polycarbonates having a large number of terminal hydroxyl groups have poor thermal stability. Further, as generally known, increasing the amount of hydroxyl groups by the use of active hydrogen compounds induces a partial chain breakage reaction to cause the broadening of the molecular weight distribution. Furthermore, relatively large amounts of catalysts are required to obtain a sufficient reaction rate, and such heavy use of catalysts can possibly cause a decrease in properties during a forming process.
Several polycarbonate production processes involving the addition of diol compounds to the reaction system have been proposed. For example, Japanese Patent Publication No. H06-94501 discloses a process for producing a high-molecular polycarbonate by the introduction of 1,4-cyclohexanediol. In the process disclosed, 1,4-cyclohexanediol is added to the polycondensation reaction system together with an aromatic dihydroxy compound at an initial stage of the reaction. It is therefore considered that 1,4-cyclohexanediol is consumed (oligomerized) first in the polycarbonate-forming reaction and thereafter the oligomer is reacted with the aromatic dihydroxy compound to attain a high molecular weight. Thus, the reaction time tends to be relatively long and appearance properties such as hue tend to be poor.
Japanese Patent Application Kokai Publication No. 2009-102536 discloses a polycarbonate production process involving the copolymerization of a specific aliphatic diol and an ether diol. However, the polycarbonates disclosed in this publication have an isosorbide-based structure and thus fail to achieve high impact resistance required for aromatic polycarbonates.
Other approaches that have been proposed include the addition of cyclic carbonate compounds to the reaction system (for example, Japanese Patent No. 3271353), and the addition of diols having a hydroxyl group basicity equal to or higher than that of a dihydroxy compound used in the reaction (for example, Japanese Patents Nos. 3301453 and 3317555). However, any of these processes do not afford highly polymerized polycarbonate resins having fully satisfactory properties.
As discussed above, the conventional processes for the production of highly polymerized aromatic polycarbonates have many problems. Thus, demands still remain for an improved production process that can increase the molecular weight to a sufficient level while maintaining the good quality inherent to polycarbonates.
The present inventors have already developed a novel process capable of a high polymerization rate and of affording a quality aromatic polycarbonate. Specifically, the process is such that end-capped terminals of aromatic polycarbonate molecules are interconnected via an aliphatic diol compound to extend the chain length (WO 2011/062220). According to this process, capped ends of aromatic polycarbonate molecules are interconnected via an aliphatic diol compound to extend the chain length so that a highly polymerized aromatic polycarbonate resin having a weight average molecular weight (Mw) of about 30,000 to 100,000 may be produced in a short time. The ability to produce polycarbonates by polymerization reaction at a high rate suppresses the occurrence of branching and crosslinking reactions which are induced by factors such as prolonged exposure to high temperatures and also makes it possible to prevent resin degradations such as poor hue.
Further, the present inventors have proposed a process for producing a branched aromatic polycarbonate resin having a desired degree of branching, the process including a step of transesterifying an aromatic polycarbonate prepolymer having a branched structure with an aliphatic diol compound in the presence of a transesterification catalyst under reduced pressure conditions (WO 2012/108510).
Furthermore, the present inventors have proposed a polycarbonate copolymer which includes structural units derived from an aromatic polycarbonate prepolymer and structural units derived from an aliphatic diol compound (WO 2012/157766).
The conventional processes for the production of highly polymerized aromatic polycarbonates have many problems. Thus, there have been demands for the development of polycarbonate resins having a sufficiently high molecular weight while maintaining the good quality inherent to polycarbonates, and for the development of processes that can produce such highly polymerized polycarbonate resins.